Electrostatic shield for inductive windings



Nov. 22, 1955 G. 1. JOHNSTON 2,724,735

ELECTROSTATIC SHIELD FOR INDUCTIVE WINDINGS Filed July 14, 1951 m N 1 4k\\\\\\\\\\\\\\\\\\/ United States Patent O ELECTRGSTATIC SHIELD FORINDUCTIVE WINDINGS Gorman J. Johnston, Pittsburgh, Pa., assigner toAllis- Chalmers Manufacturing Company, Milwaukee, Wis.

Application July 14, 1951, Serial No. 236,794

10 Claims. (Cl. 174-35) This invention relatesto electrical inductionapparatus and, in particular, to electrostatic shields for the windingsof such apparatus.

As is well known, stationary induction apparatus such as transformersmay include electrostatic shields which distribute surge voltages byelectrostatically imposing the surge voltage relatively uniformly acrossthetirst coil of the winding of such apparatus as a result of the closeelectrostatic coupling between the shield and the first coil. The surgevoltages otherwise may be almost entirely carried by the first turn ofthe coil of the winding, imposing very high voltage stresses on thatfirst portion of the coil. Electrostatic shields formed as rings ofconductive material are open circuited in order that short circuitcurrents may not be induced therein by the magnetic flux. It has beenproposed that the conductive plate or ring of these shields be made fromone piece or strip of conductive material, either a tiat ring of solidmaterial or a spiral winding of a noninsulated conductor forming a diskcoil similar to the coils of the windings of the transformer. In eithercase, of course, the shield is insulated from the other conductive partsof the apparatus except for the terminal lead to which it is electricalyconnected.

in order to .more efficiently produce electrostatic shields and at thesame time provide an improved electrostatic shield for embodiment intransformers, in accordance with this invention, the shield isfabricated and includes a supporting form of insulation material, abraided sleeve of conductive material adapted to and firmly secured tothe supporting form, and insulating elements which prevent physicaldistortion of the shield from causing abutment of the ends of theconductive material at the gap in the shield.

The primary object of this invention is to provide an improvedelectrostatic shield for stationary induction apparatus.-

Another object of this invention is to provide a fabricatedelectrostatic shield. l

Still another object is to make components of an electrostatic shield sothat the shield can be fabricated by relatively simple shop handling yetinsuring a strong shield in which the assembly of components positivelyinsures a permanent gap in the electrostatic shield.

lt is still another object to provide anelectrostatic shield constructedof components which can be fabricated utilizing relatively simple shoppractices with the fabricated components making a substantially smoothsurfaced finished electrostatic shield.

Objects and advantages other than those aboveset forth will be apparentin the following description when read in connection with theaccompanying drawing, in which:

Fig. l is a longitudinal view 'in section. of part of a transformerembodying an electrostatic shield constructed in accordance with thisinvention;

Fig. 2 is an enlarged plan view of the electrostatic shield shown inFig. 1 with parts of the shield broken away.

Figs. 3 to 8 are enlarged views in cross section across the gap in theelectrostatic shield showing component parts. in` particular, Fig. 3 isa plan View showing the insulating ice form for the braided conductivesleeve of the shield, and Fig. 4 is a section of the member shown inFig. 3. Fig. 5 is a view in section of the insulating form of Fig. 4with its enclosing braided conductive sleeve. Figs. 6 to 8 are sectonalviews similar to Fig. 5 showing how the insulating components of the gapare added in the electrostatic shield.

Fig. 9 is an enlarged sectional view taken along line IX-IX of Fig. 2with the outside insulating tape removed.

ln Fig. l the portion of the transformer shown in section includes thecore of which the member 11 represents a leg around which are thewindings l2 and 13. The high voltage winding 12 has disk or pancakecoils which have conventional insulation separating them from each otheras well as from the core of the transformer. Associated with the highvoltage winding is the electrostatic shield 15.

The shield represented in Fig. l is better shown in the other figures ofthe drawing from which it can be seen that the component parts thereofare so constructed that the shop assembly or fabrication of the shieldmay be relatively easily accomplished.

in particular, the shield is made of any suitable conductive materialsuch as copper. An improved, easily constructed and assembled conductivemember for the shield is provided by utilizing braided copper materialre ducing the amount of conductive material necessary for making thesplit ring and also avoiding the winding time and waste of materialresulting from making the split ring by spirally winding a noninsulatedconductor and then cutting an air gap therein.

ln this instance, the iiexible braid is given its circular form or shapeby a ring-shaped insulating form 21 inserted inside the braid. A sleeveof braided copper 20 is pulled over the insulating form 2i. Then thesleeve is cut to the proper length and fastened to the insulating form.

ln the illustrated example, the insulating form is especiallyconstructed so that the ends of the braided sleeve may be securelyfastened thereto. The construction of the end portions of .theinsulating form is such that the spaced ends of the braided sleeve atthe air gap will be substantially permanently separated and insulated.

The end portions of the insulating form are step shaped, one end, thatshown on the left hand side in Figs. 3 and 4, having its upper facestepped while the right hand end portion has its lower face stepped. Inthe preferred embodiment this step shaped construction is accomplishedby providing a pair of split insulating rings 28, 29 of identical sizeand shape of suitable material such as tullerboard. One of theinsulating rings, 28, is placed over the other and may be glued theretowith the gaps in the two rings circumferentially offset a distance lessthan the width of the gaps, to provide the insulating form shown inFigs. 3 and 4 with its step-shaped spaced end portions.

The reduced cross sectional portions adjacent the ends of the insulatingform 21 serve several purposes and make it possible to well constructthe electrostatic shield at the gap and end portions adjacent thereto.For ex ample, 'for best results it is desirable to stretch the braidedsleeve when pulling it over the form to make it apply itself tightly tothe form. To securely fasten the stretched sleeve and to insure that thesleeve is uniformly stretched over the insulating form, each end of thebraided sleeve is pulled beyond the corresponding end of the form. Then,the extending end portions of the sleeve are folded back on thecorresponding end portions of the form to which they are fastened bysuitable means. The folded portions of the braided sleeve fit into theend portions of reduced thickness of the form 2l. With the sleevestretched and pulled over the end of the form, simple fastening meanssuch as rivets may be utilized and the sleeve being folded back asdescribed will be kept stretched over the end of the form. By stretchingthe braid over the end of the form, a smooth and even conductive surfacedefines the ends of the conductive ring at the gap in the shield. Thegap can be accurately de termined and no loose ends of conductive braidwill be able to bridge the gap. The portions of the braided sleeve whenfolded back on the form will not in this instance increase the thicknessof the shield because the end portions of the insulation form havesufficiently reduced thickness to accommodate the folded braid.

The shield further includes insulating components cooperative with thestructure hereinbefore described by which the ends of the conductivebraid are insulated and the gap set in a mode facilitating the shopassembly of the electrostatic shield. In particular, there is providedan insulating strip 23 which may be fastened to the under face of thebraided ring and which bridges the air gap. This strip 23 is longer thanthe distance between the steps on the end portions of the insulatingform, and the rivets 25 extend through both layers of the insulatingform and through the insulating strip 23. The desired air gap can beeasily and accurately set when one end part or portion of the longinsulating strip 23 has been riveted to the form and before the otherend part of the strip 23 has been riveted in position.

As can be seen in Fig. 6, there is clearance between the insulatingstrip 23 and the copper braid in the area corresponding to the righthand side reduced portion of the insulating form 21. One end part orportion of a crossover insulating strip 22 is inserted into thatclearance space. The other end part of the crossover strip 22 rests onthe upper face of the folded braid on the left hand side of the gap.When the crossover strip is properly positioned, it is also fastened tothe braid and insulating form preferably by rivets 26. After thecrossover strip is fastened in position, a third insulating strip, 24,is placed across the upper face of the ends of the braided sleevebridging the gap to add mechanical strength to the connection betweenthe ends of the shield. Suitable means are provided to hold this upperstrip, 24, to the shield; in this instance the strip 24 is held in placeby gummed paper tape (not shown) which also will strengthen theconnection between the ends of the shield.

The crossover strip being attached at its opposite ends to the oppositefaces of the braid and the intermediate portion of the strip crossingthe gap between the ends of the braid provides positive insulationseparating the ends of the conductive braid and also firmly maintainingthe proper gap. Under some conditions the assembly of the transformermay result in some wires of the braided sleeve loosening or in theconductive braid being disturbed so that conductive material may tend tobridge the gap. However, with the crossover insulation herein provided,the gap cannot be bridged by any of the con ductive material.

The terminal lead 31 shown enlarged in section in Fig. 9 is attached tothe shield at a point diametrically opposite the insulated gap.Preferably, the lead 31 is braided copper, and the end thereof issuitably attached to the shield, in this instance by rivet 30.Preferably the lead is pulled radially outwardly from the shield afterhaving been fastened to the upper face of the braided sleeve, pulleddown the inner edge and radially across the lower face of the braidedsleeve. The lead 31 is covered with insulation material 32, such ascrepe paper tape.

After the terminal lead is attached the entire shield is insulated.Insulation, such as cable paper 33, is wrapped around the braidedsleeve, ring-shaped element of the shield by an overlapping wrappingoperation. The number of layers of insulation paper depends on therequired dielectric strength. Then a layer of protective cotton tape 34is wrapped over the insulation paper thereby increasing the mechanicalstrength of the wrapping.

Although but one embodiment of the present invention has beenillustrated and described, it will be apparent to those skilled in theart that various changes and modifications may be made therein withoutdeparting from the spirit of the invention or from the scope of theappended claims.

It is claimed and desired to secure by Letters Patent:

l. An electrostatic shield comprising a ring shaped insulating formhaving a gap, a braided sleeve of con ductive material, said forminserted into said sleeve with the end portions of said sleeve spacedfrom each other at said gap, and means insulating the end portions ofsaid sleeve from each other including an insulating strip having one ofits end portions over a first end portion of said sleeve and having theother of its end portions under the second end portion of said sleevewith said strip crossing over from the upper face to the lower face ofsaid sleeve at said gap between the ends of said sleeve.

2. An electrostatic shield comprising a ring shaped insulating formhaving a gap, a braided sleeve of conductive material, said forminserted into said sleeve with the end portions of said sleeve spacedfrom each other at said gap, and means for insulating the end portionsof said sleeve from each other including a plurality of insulatingstrips, a first of said strips bridging said gap and fastened to thelower faces of both end portions of said sleeve, a second of said stripsbridging said gap between the ends of said sleeve with the opposite endsof said second strip on opposite faces of said sleeve, a third of saidstrips bridging said gap and fastened to the upper faces of both endportions of said sleeve.

3. An electrostatic shield comprising a ring shaped insulating formhaving a gap, a braided sleeve of conductive material, said forminserted into said sleeve with the end portions of said sleeve spacedfrom each other at said gap, and means for insulating the end portionsof said sleeve from each other including a plurality of insulatingstrips, a first of said strips bridging said gap across the lower facesof both end portions of said sleeve, a second of said strips bridgingsaid gap and crossing between the ends of said sleeve with the oppositeends of said second strip on opposite faces of said sleeve.

4. An electrostatic shield comprising a ring shaped insulating formhaving a gap between its spaced ends, a braided conductive sleeve intowhich said form is inserted, end portions of said sleeve extendingbeyond the ends of said form and being folded back over thecorresponding end portions of said form, said sleeve at the foldsforming spaced ends of a split conductive ring, and means insulatingsaid end portions of said sleeve including an insulating strip havingone of its end parts over a first of said end portions of said sleeveand having the other of its end parts under the second of said endportions of said sleeve.

5. An electrostatic shield comprising a ring shaped in sulating formhaving a gap, the end portions of said form having reduced thickness, afirst end portion of said form being stepped on its upper face and asecond end portion of said form being stepped on its lower face, abraided conductive sleeve into which said form is inserted, portions ofsaid sleeve extending beyond the ends of said form being folded overcorresponding ends of said form, one of said folded portions of saidsleeve being disposed over said first portion of said form and beingconfined to the area of reduced thickness thereof, the other of saidfolded portions of said sleeve being disposed under said secondendportion of said form and being confined to the area of reduced thicknessthereof.

6. An electrostatic shield comprising a form including a pair ofsubstantially identical ring shaped insulating e1ements each having agap, one of said elements being laid over the other of said elementswith the gaps in said two elements being circumferentially offset adistance less than the width of said gaps to provide said form withfirst and second step shaped end portions circumferentially spaced fromeach other, a braided conductive sleeve into which said form isinserted, end portions of said sleeve extending beyond the ends of saidform, a rst end portion of said sleeve being folded over one end of saidform and confinetd to said first step shaped portion and the other endportion of said sleeve being folded under the other end of said form andbeing conned to said second step shaped portion, and means forinsulating from each other the said end portions of said sleeve.

7. An electrostatic ring comprising a form including a pair ofsubstantially identical flat ring shaped insulating elements each havinga gap, one of said elements being laid over the other of said elementswith said elements circumferentially offset a distance less than thewidth of said gaps to provide said form with spaced first and second endportions of reduced thickness stepped on opposite faces of said form, abraided conductive sleeve into which said form is inserted, said sleevebeing longer than said form, end portions of said sleeve folded at theends of said form, one folded end portion of said sleeve being conlinedto the reduced thickness area of said rst end portion and the otherfolded end portion of said sleeve being confined to the reducedthickness area of said second end portion, and an insulating stripbetween the ends of said sleeve, one end of said strip extending overthe upper face of said sleeve and the other end of said strip extendingunder the lower face of said sleeve.

8. An electrostatic shield comprising a ring shaped conductive memberhaving a relatively short axial dimension and a relatively long radialdimension with a gap between adjacent ends thereof, said ends lyingwithin the radially spaced edges and within the axially spaced faces ofsaid member, means for preventing flow of current between said ends byway of said gap comprising insulating means between said ends includingan insulating strip conlined within said edges and within said faces,said strip having one of its end parts over a first end portion of saidmember and having the other of its end parts under the second endportion of said member with its intermediate part extending transversesaid ends, and fastening means securing said insulating means to saidmember.

9. An electrostatic shield comprising a ring shaped conductive memberhaving a relatively short axial dimension and a relatively long radialdimension with a gap between adjacent ends thereof, the end portions ofsaid member lying within the radially spaced edges and within theaxially spaced faces of said member, means for preventing the flow ofcurrent between said end portions by way of said gap comprisinginsulating means between said end portions including a plurality ofinsulating strips, a rst of said strips confined within said edges andwithin said faces of said member bridging said gap and crossing betweensaid ends with the opposite end parts of said first strip fastened toopposite faces of said end portions, a second of said strips bridgingsaid gap and fastened to the lower faces of said end portions, and athird of said strips bridging said gap and fastened to the upper facesof said end portions, and fastening means securing said insulating meansto said member coacting with the end portions of said member and saidinsulating strips to hold said ends in place and to maintain the propergap.

10. An electrostatic shield comprising a ring shaped conductive memberhaving a relatively short axial dimension and a relatively long radialdimension with a gap between adjacent ends thereof, end portions of saidmember lying within the radially spaced edges and within the axiallyspaced faces of said member, means for preventing the flow of currentbetween said ends by way of said gap including a plurality of insulatingstrips, a first of said strips confined within said edges and withinsaid faces of said member bridging said cap and crossing between saidends with the opposite end parts of said rst strip being on oppositefaces of said end portions, a second of said strips bridging said gapand fastened to the common faces of said end portions, and fasteningmeans securing said insulating means to said member coacting with saidend portions and said insulating strips to hold said ends in placemaintaining the proper gap therebetween.

References Cited in the file of this patent UNITED STATES PATENTS1,942,575 Shapiro Ian. 9, 1934 2,280,137 Wiegand Apr. 21, 1942 2,515,333Buflington July 18, 1950

